1. Field of the Invention
The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube apparatus having an in-line type electron gun assembly which can compensate for static misconvergence of three electron beams, caused by fluctuations in focus of the electron beams.
2. Description of the Related Art
An in-line type electron gun assembly in a conventional color cathode ray tube apparatus as shown in FIG. 1 comprises cathodes 2 respectively incorporating heaters 1, and the following grids, each of which is integrally formed: a first grid 3, a second grid 4, a third grid 5, and a forth grid 6. The third grid 5 is constituted by a cylindrical member having a bottom, which is integrally formed by a mechanical means. Apertures 5G, 5B, and 5R are formed in the bottom of the cylindrical member such that the centers of the apertures respectively coincide with a gun axis ZG of the center electron gun and the gun axes ZB and ZR of the side electron guns. Similarly, the forth grid 6 is constituted by a cylindrical member having a bottom, which is integrally formed by a mechanical means. Apertures 6G, 6B, and 6R are formed in the bottom of the cylindrical member such that the center of the aperture 6G coincides with the gun axis ZG, and the centers of the apertures 6B and 6R are respectively eccentric from the gun axes ZB and ZR. A main electron lens L110 is formed between the third grid 5 and the forth grid 6.
According to such an electron gun assembly, as disclosed in Published Examined Japanese Patent Application No. 52-32714, in the center electron gun, since the centers of the apertures 5G and 6G coincide with the gun axis ZG, a center electron beam 9G propagates straight ahead to a phosphor screen (not shown). In contrast to this, in the side electron guns, each electric field is formed to be asymmetrical about a corresponding one of the gun axes ZB and ZR, and side electron gun beams 9B and 9R passing through these electric fields are bent toward the center electron beam 9G. As a result, these three electron beams 9B, 9G, and 9R are caused to converge on the phosphor screen. As disclosed in Published Examined Japanese patent Application No. 53-38076, electrodes having inclined apertures are used to form asymmetrical electric fields.
In an electron gun assembly, the structure of each electrode is mechanically simple, and the relative positions of the electron lenses of the three electron guns can be accurately determined. Therefore, an electron gun assembly is advantageous in terms of cost and precision. However, there is room for further improvement in such an electron gun assembly. That is, a feature to be improved is associated with eccentrically formed or inclined apertures which are used to converge three electron beams at a predetermined position. The deflection amount of an electron beam deflected by an asymmetrical electron lens formed by such an eccentrically formed or inclined aperture is approximately proportional to the eccentricity or inclination of the aperture and the difference in potential between electrodes which form the electron lens. More specifically, the deflection angle (amount) of a beam deflected by an asymmetrical electron lens is approximately given by the following equation: EQU .theta.=k.multidot.p.multidot.q (1)
where .theta. is the deflection angle, k is a constant, p is a value obtained by normalizing an electron lens diameter with an eccentricity amount, and g is the voltage ratio of the electron lens.
If, therefore, a voltage is inaccurately applied between the electrodes which form the electron lens, the deflection angle .theta. is changed. As a result, static convergence of a color receiver set with no deflection magnetic field being applied is deviated. For example, in an electron gun using a bipotential type electron lens (Bi Potential Focus: to be referred to as a BPF hereinafter), a high acceleration voltage of 25 to 32 kV is applied to the forth grid, and an intermediate voltage set to be 25 to 35% of a convergence voltage is applied to the third grid. However, a voltage to be actually applied includes an error of .+-.1% of the intermediate voltage due to assembly errors of the associated components. In consideration of convergence, this error is too large to be neglected.
Especially in a recent color cathode ray tube apparatus, final adjustment of a cathode ray tube is performed before it is mounted in a receiver set. For example, Published Examined Japanese Patent Application No. 51-45936 discloses a preset type cathode ray tube, in which three axes, i.e., the tube axis, the axis of an electron gun axis, and the axis of a deflection device are matched with each other by adjusting the field intensity of a permanent magnet magnetized to a plurality of poles and mounted on the outer surface of the neck of a vacuum envelope of the cathode ray so that no adjustment is required after the cathode ray tube is mounted in the receiver set. In a cathode ray tube of this type, as described above, especially when the difference in potential between the electrodes which form an electron lens requires accuracy, if operation conditions of each electron gun, especially a voltage to be applied to the third grid 5, are inaccurately set in adjustment of the receiver set, the electron gun assembly must be adjusted again after it is mounted in the receiver set. This leads to a deterioration in operation efficiency.
Several means for solving such a problem associated with a change in focusing electric field have been proposed. For example, as shown in FIG. 2, Published Examined Japanese Patent Application No. 1-42109 discloses a structure in which first electron lenses are formed between a third grid 5, a forth grid 6, and a fifth grid 7, and second electron lenses are formed between the fifth grid 7 and a sixth grid 8 in such a manner that apertures which oppose each other are eccentrically formed to make the first and second electron lenses asymmetrical, through which side beams pass to be deflected to converge at a predetermined position. In such a structure, however, a side electron beam deflected by the first electron lens propagates along the tube axis side of the second electron lens and hence is subjected to the influence of a coma through the second electron lens. As a result, a halo may be produced in the side electron beam in a lateral direction.
Published Unexamined Japanese Patent Application No. 55-37798 discloses a structure in which an electron gun constituted by asymmetrical first and second electron lenses L110 and L120 is designed such that a side electron beam deflected by the first electron lens L110 is incident on the second electron lens L120 while it is substantially inclined to its center, and apertures are eccentrically formed in opposite electrode which form the second electron lens L120. In this structure, however, the structure of each electrode is complicated, and the number of types of electrodes is increased. Therefore, it is very difficult to assemble the electrodes of each electron gun with high precision. This may decrease the resolution.
In addition, Published Unexamined Japanese Patent Application No. 1-42109 or 55-37798 discloses an arrangement in which first and second electron lenses L110 and L120 serve to not only deflect a side electron beam in the in-line direction but also focus it in a direction perpendicular to the in-line direction. FIG. 3 illustrates a positional relationship between an electron lens system and object points in this arrangement. When the first electron lenses L110 for correcting convergence are neglected, electron beams emitted from virtual object points VP located on the respective axes are focused to a predetermined position by the second electron lenses L120. In practice, however, since the first electron lenses L110 have focusing effects, the virtual object points VP are formed before and after predetermined positions. Especially, since each first electron lens L110 is an asymmetrical electron lens, an electron beam incident on a corresponding second electron lens L120 has an astigmatism. Since an object point viewed from each second electron lens L120 is distorted and deteriorated, a spot size on a phosphor screen is increased, resulting in a decrease in resolution.